[Python-Dev] PEP 498 (interpolated f-string) tweak

[Serhiy Storchaka]

On 20.09.15 16:51, Eric V. Smith wrote:
> On 9/20/2015 8:37 AM, Nick Coghlan wrote:
>> On 19 September 2015 at 21:03, Eric V. Smith <eric at trueblade.com> wrote:
>>> Instead of calling __format__, I've changed the code generator to call
>>> format(expr1, spec1). As an optimization, I might add special opcodes to
>>> deal with this and string concatenation, but that's for another day (if
>>> ever).
>>
>> Does this mean overriding format at the module level or in builtins
>> will affect the way f-strings are evaluated at runtime? (I don't have
>> a strong preference one way or the other, but I think the PEP should
>> be explicit as to the expected behaviour rather than leaving it as
>> implementation defined).
>
> Yes, in the current implementation, if you mess with format(), str(),
> repr(), or ascii() you can break f-strings. The latter 3 are used to
> implement !s, !r, and !a.
>
> I have a plan to change this, by adding one or more opcodes to implement
> the formatting and string joining. I'll defer a decision on updating the
> PEP until I can establish the feasibility (and desirability) of that
> approach.

I propose to add internal builting formatter type. Instances should be 
marshallable and callable. The code generated for f-strings should just 
load formatter constant and call it with arguments. The formatter builds 
resulting string by concatenating literal strings and results of 
formatting arguments with specified specifications.

Later we could change compiler (just peephole optimizer?) to replace 
literal_string.format(*args) and literal_string % args with calling 
precompiled formatter.

Later we could rewrite str.format, str.__mod__ and re.sub to create 
temporary formatter object and call it.

Later we could expose public API for creating formatter object. It can 
be used by third-party template engines.